Laser device for producing multiple frequency outputs



March 11, 1969 R. L. GARWIN 3,432,768

LASER DEVICE FOR PRODUCING MULTIPLE FREQUENCY OUTPUTS Filed June 26,1964 Sheet of 5 F'LGJ 2 J I ABSOR- 6 LASER BENT 4h ACTIVE MEDIUM AT c 1/3% Q FIG. 2

EXCITATION LEVEL I3 FIG.3

NON- RADIATI VE METASTABLE LEVEL RADIATIVE 14 lNvENToR.

NON-RMTIVE LEVEL "A TERM RICHARD L. GARWIN March 11, 1969 R. L. GARWIN3,432,763

LASER DEVICE FOR PRODUCING MULTIPLE FREQUENCY OUTPUTS Filed June 26,1964 Sheet 2 of 5 FIG. 4

hI/c hZ I VIRTUAL LEVELS hI/C a FIG. 5

RELATIVE TRANSITION PROBABILITY I I l l 0 2 .4 .6 s I 0 wA/wB *& -I|l 102 17 16 2 5 II 8 fi fim fiW FIG. 6 8 L I I I I I 1/ March 11 1969 G wm3,432,768

LASER DEVICE FOR PRODUCING MULTIPLE FREQUENCY OUTPUTS Filed June 26,1964 Sheet 3 of 5 I flkl c v FIG. 7 POWER A OUTPUT UEs/N w) FIG. 9

POPULATION t (NANOSECONDS) United States Patent ABSTRACT OF THEDISCLOSURE The invention relates to a laser device which is capable ofproviding output at at least two different frequencies, the sum of whichis equal to the characteristic frequency of a particle within a laseractive medium which has a forbidden single quantum transition.

The coherent light output from optically pumped solidstate lasers in theform of intense, sharp, single pulses is commonly referred to as a giantpulse because their peak intensity is greatly in excess of the peaks ofthe normally emitted light from a laser which has been pumped beyondthreshold. While the normal output of a laser has a duration of severalhundred nanoseconds, the giant pulse variety produces sharp, intensespikes of coherent light which have durations of some tens ofnanoseconds. A number of well-known techniques for producing giantpulses can be found in the literature. In one usual case, anelectro-optical shutter is interposed between a totally reflectingsurface and the end of the active material of a laser. The generation ofa giant pulse is initiated by closing the shutter and applying a pulseto an energizing lamp which surrounds the laser material. When thepopulation inversion required for normal laser oscillation is greatlyexceeded (the feedback loop is open due to the fact that theelectro-optical shutter is closed), the shutter is opened andregeneration occurs at a high rate because of the abnormally high gainin the material. Energy which has been stored in the laser is releasedpractically instantaneously and a high peak-power transient occursresulting in the well-known giant pulse output. More recently,techniques have been disclosed which show the utilization of Kerr cellsfor Q switching to produce giant pulses. Other techniques incorporatingQ-switching using special Q-switching elements have also been disclosed,but the basic mechanism for producing giant pulses in lasers of thistype remains the same.

In a more recent article in the IBM Journal of Research and Development,volume 8, number II, April 1964, an article by P. P. Sorokin and N.Braslau, titled Some Theoretical Aspects of a Proposed Double QuantumStimulated Emission Device, another technique for producing giant pulsesis disclosed. In this article, a coherent light generator making use ofdouble-quantum stimulated emission is discussed. In the arrangementdisclosed, a laser crystal having conventional laser geometry is dopedwith two fluorescent io-ns A and B. The A and B ions of this arrangementbear a relationship to each other such that the frequency of the laserlight generated by the A ion when multiplied by two falls within thespontaneous emission line width of a B ion fluorescent transition whichcan be inverted by the action of the same energizing lamp which acts onthe A ions. A relationship is further established such that v =2v where11 =the frequency of the B ion, and v =the frequency of the A ion.

Another criterion is that lasing action does not take place at 1 Thiscriterion can be satisfied by providing a low 3,432,768 Patented Mar.11, 1969 by strong parasitic absorption in the laser material nearfrequency 113 or preferably by a choice of ion such that the transition11 is highly forbidden to a single-quantum process. In addition to thearticle referred to above, subject matter related to the presentapplication is disclosed in a copending patent application, entitledMultiple Photon Laser, filed on behalf of Peter P. Sorokin on Apr. 10,1964, having the Ser. No. 358,733, and assigned to the same assignee asthe present application.

The device of the article is energized by a flash lamp which pumps boththe A and B ions, inverting the population of these ions, and byproviding a high cavity Q at frequency 11 Oscillations occur at thisfrequency and the characteristic output of an optically pumpedfour-level solid-state laser is obtained. Large photon populations arethus sporadically produced in certain modes having frequencies which lienear the peak 11 of the A-ion fluorescence. No stimulated emissioninvolving the inverted B ion population can occur because of the low Qconditions established for 2 but if the photon density becomes largeenough in one of the modes coherently excited by the A ions, the crosssection becomes appreciably large for a double quantum transition inwhich a B ion is de-excited and two photons are simultaneously added tothe mode which lies near peak M of the A-ion fluorescence. Since thecross section for this process continues to increase with the additionof more photons to the aforementioned mode, an avalanche effectproducing the required photons can occur resulting in a giant pulse atthe V frequency. The pulse then continues until the population excess ofB ions is largely reduced.

In the above mentioned article, the relationship between the A ionfrequency and the B ion frequency is shown to be 11 :2 and an output atfrequency 11 is provided. A high Q is established for frequency 11 inone instance, by applying highly reflective coating at frequency v tothe end of the laser; by applying highly transmissive coating atfrequency 11 on the end of the laser and by doping the laser activematerial with a B ion having a two-photon transition at a frequency 211In the present application, it will be shown that giant pulses at afrequency M can be obtained in addition to other giant pulses at atleast another different frequency such that v +v =11 In other words, itwill be shown that other relationships can be derived from systems whichexhibit multiple quantum stimulated-emission phenomenon. Certainstructural criteria must be met, however, which will be discussedhereinbelow which permit the generation of intense, sharp coherent lightpulses at two frequencies.

It is therefore an object of this invention to provide a laser employingthe principle of multiple quantum stimulated emission which is animprovement over prior art devices.

Another object is to provide a laser which is capable of producing giantpulses of coherent light at more than one frequency.

Another object is to provide a laser device which is capable ofproducing giant pulses at more than one frequency; the outputs having arelationship v +v =v Yet another object is to provide a laser devicewhich is capable of simultaneously producing high peak-power pulses atat least two-different frequencies.

Still another object is to provide a laser device in which stimulatedemission takes place at a priming frequency and at at least anotherfrequency different from the priming frequency.

Another object is to provide a laser which makes available highintensity laser outputs in a new short wavelength range.

cavity Q at frequency 11 A feature of this invention is the utilizationof a laser active medium containing at least a single group of particlescapable of entering an excited state from which deexcitation occurs as amultiple photon transition and in which apparatus associated with thelaser active medium produces stimulated emissions at at least twofrequencies and at least a two-photon transition at frequencies whosesum is the characteristic frequency separation between the excited stateand a lower state.

Another feature of this invention is the utilization of a laser activemedium containing at least two groups of particles at least one of whichis capable of entering an excited state from which de-excitation occursas a multiple quantum transition which is energized to producestimulated emission at two frequencies and at least a two photontransition at a sum frequency which is characteristic of one of thegroups of particles.

Another feature of this invention is the utilization of a laser activemedium containing at least a single group of particles capable ofentering an excited state from which de-excitation occurs as a multiplephoton transition which incorporates an excitation lamp and a reflectiveelement optically coupled to the output of the laser active medium toreflect a broad band of frequencies; the reflective element providing ahigh Q to the stimulated emission frequencies which result from thedouble-quantum emission phenomenon.

Yet another feature of this invention is the utilization of a laserdevice having a two photon transition and first and second stimulatedfrequency components the laser device being characterized by a surfacewhich is reflective of a band of frequencies including both the firstand second stimulated emission frequency components.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiment of the invention as illustrated inthe accompanying drawings.

In the drawings:

FIG. 1 is a schematic illustration showing an aspect of the inventionutilizing a laser active medium which includes certain semiconductors,glasses, liquids, gases and insulators.

FIG. 2 is a schematic illustration showing an embodiment of theinvention utilizing a laser active material containing a single group ofparticles and a source of priming photons disposed externally of thelaser material.

FIG. 3 is an energy level diagram which indicates the varioustransitions an excited particle undergoes in passing from an excitedstate to its normal ground state.

FIG. 4 is an energy level diagram for a laser exhibiting the doublequantum transition phenomenon.

FIG. 5 is a plot of the Relative Transition Probability vs. w /w showingthe probability for the occurrence of a two photon transition.

FIG. 6 is a schematic illustration of an embodiment of this inventionwhich utilizes Kerr-cell Q-switching to provide a giant pulse atfrequency 11 to prime a laser active medium containing a single group ofparticles.

FIG. 7 is a plot of energy vs. time showing the relative timing ofevents during the production of a giant pulse.

FIG. 8 is a plot of the number of photons of either frequency 11 or w asa function of time in the development of the giant pulse.

FIG. 9 is a plot of population vs. time showing the course of events inthe nondegenerate two-photon laser of this system.

The novel features of the present invention arose from the recognitionthat, theoretically, no restraint need be placed on the relationshipbetween 11 and 11 The concept that the giant pulse outputs of a laser attwo frequencies as defined by the relationship gives wider applicationto the double-quantum stimulated emission phenomenon as set down in theabove mentioned article wherein the relationship that VB=2IIA was setforth for obtaining a giant pulse output.

The criterion that the B-ion frequency VB be twice the A-ion frequencydoes not have to be adhered to except in the special case where amaximum power pulse at a single frequency is required. The novelty ofthe present invention resides in certain structural features whichdistinguish it over both the prior art and devices suggested by theabove mentioned article. The novel structural features; functions andresults of this invention are detailed in the following discussion;

In FIG. 1, a laser active medium 1 is shown surrounded by energizingflash lamp 2. Laser active medium 1, in its broadest aspect, is a cavityresonant at frequencies VA and V0 which contains N ions per cc. of typeA and N ion per cc. of type B in a host material. Thus, laser activemedium 1 could be any one of a number of solid, liquid, or gaseousmaterials containing ions, atoms or molecules with narrow electronicenergy levels which provide light amplification by stimulated emissionof radiation. Such ions, atoms, and molecules of type A can be definedas particles capable of entering an excited state from whichde-excitation occurs as a single quantum transition, while type B can bedefined as particles capable of entering an excited state from whichde-excitation occurs as a multiple quantum transition. Thus, wherereference is made hereinafter to an A particle or a B particle, itshould be understood that such particles include atoms, ions, andmolecules. As is well known, certain glasses appropriately doped, aswell as insulators and semiconductor materials are included among thematerials which provide stimulated emission light outputs. It shouldalso be ap preciated that energizing flash lamp 2 is not the only meansfor inverting the ion populations present in laser active media. In FIG.1 a flashlamp 2 has been shown for purposes of explanation but,depending upon the type of laser active medium, any other means ofpopulation inversion can be utilized in the practice of this invention.A chemical reaction which provides products in states of electronicexcitation might prove most appropriate where the laser active medium isa gas. Population inversion may also be obtained electrically by meansof a discharge through a gaseous medium or by carrier injection wheresolid state semiconductors are utilized.

A detailed analysis to be disclosed hereinafter shows that laser activemedium 1 may be primed at frequency u with a number of photons smallcompared with N and will provide giant pulses simultaneously at 11 and11 in accordance with the relationship that v =v +v The relaxation ofthe requirement that v =2v allows the use of metastable levels 11 suchthat 1/ is very much greater than 11 (in excess of 211 and makesavailable high-intensity laser outputs in a new short-wave length range;allows the production of new laser lines rather than amplification ofknown ones and eases substantially the problem of designing a systemwhich exhibits the unique high output power characteristics of themultiple photon laser.

Referring again to FIG. 1, laser active medium 1 has a filter element 3optically coupled to the ends thereof. Element 3 may be depositeddirectly on the ends of laser active medium 1 or may be spaced from theends without changing its function in any way. Filter element 3 mayassume two configurations; both of which are shown in FIG. 1. At theleft hand of laser active medium 1, filter element 3 consists of areflecting layer 4 which has a broad band reflection capability suchthat it is simultaneously reflective at frequencies 11 and 11 Layer 4further has the property of being transmissive at the B particlefrequency. At the right hand end of medium 1, filter element 3 consistsof an absorbing medium 5 interposed between medium 1 and a plurality ofreflecting layers 6 which are reflectors at frequencies 11 and wrespectively. Absorbing medium 5 is absorptive at the B particlefrequency and causes a low cavity Q at the B- particle frequency. Thereflecting layers 6 each have individual characteristics such that apeak reflectivity is attained at frequencies 11 and v In general, filterelemen 3 may consist of layers of dielectric material having nar row orbroad pass and stop characteristics or may be metallic reflectors orother arrangements well known to those skilled in the optics art as longas a high Q is simu taneously present at frequencies VA and 11 Laseractive medium 1 in FIG. 1, which may be, for example, calcium fluoridedoped with divalent thulium and dysprosium acting as the A and B ions,respectively, operates in the following manner. Flash lamp 2 surroundinglaser active medium 1 is triggered from a source not shown. The energysupplied from flash lamp '2 pumps both the A and B particles present inlaser active medium 1 causing a population inversion of each of theparticles. The A and B particles are thus removed from their normalground state by addition of energy from flash lamp 2. The particles areelevated to a higher energy level at which they are relatively unstableand from which they tend to return to their normal stable ground state.In returning to the normal ground state, considering for a moment onlythe A particles, energy is given up and stimulated emission at the Aparticle frequency occurs provided oscillations within medium 1 havebeen attained by reflections from filter element 3 at the A particlefrequency. Pulses of light characteristic of optically pumped solidstate lasers are then emitted at frequency 11 Considering now whathappens to the B particles, it should be recalled that filter element 3is highly transmissive or absorptive at the B-particle frequency. Undersuch circumstances, oscillations will not occur and there is nostimulated emission at the B-particle frequency. Large photonpopulations, however, are available due to the presence and stimulatedemission of the A particles. There is also a considerable populationavailable at frequency 11 The laser active medium 1 of FIG. 1, it shouldbe recalled, is reflective or has a high-cavity Q at frequency v Becauseof the low cavity Q due to the transmissivity or absorptivity at theB-particle frequency, no stimulated emission at that frequency occurs,but a condition is created such that if the photon density becomes largeenough, the possibility for the occurrence of a double-quantumtransition (also called a two-photon transition) in which the B particleis de-excited and in which two photons are added to the systemincreases. Since the probability for the occurrence of a double quantumtransition increases with the addition of more photons to the system, anavalanche effect is produced by the addition of the photons and a giantpulse results at frequencies 11 and w the summation of which is equal tothe B-particle frequency, 11

In FIG. 2, another laser device incorporating. a laser active medium 1containing only a single'group of particles, B particles, for instance,is shown. Laser active medium 1 may be a single-crystal material, forinstance; which is appropriately doped with B-type particles. Also shownare means for energizing the laser device to produce stimulated emissionat at least two frequencies and a two photon transition at frequenciesthe sum of which is the characteristic frequency separation between theexcited state and a lower state. Included in the above mentioned meansis an excitation lamp 2 surrounding laser active medium 1 and aconventional laser device 7 of a host material doped with appropriate Aparticles also surrounded by excitation lamp 2. Laser device 7 has twototal internal reflective surfaces 8 at one end thereof and another end9 which is untreated. A filter element 3 is shown disposed adjacent theright hand end of laser active medium 1. Filter element 3, as in FIG. 1,may consist of a broadband reflecting layer which is reflective at 11and v and transmissive at frequency 11 or may consist of separatereflecting layers 10, 11 for each of the frequencies 11 and 11respectively, in addition to an absorbing medium 5 which is highlyabsorptive at 1/ The system of FIG. 2 is similar to an embodimentdisclosed in the aforementioned copending application with the exceptionthat the reflective layers of filter element 3 are reflecitve at twodifferent frequencies rather than one. In the arrangement disclosed inthe copending application, priming photons at the A-particle frequencyare provided from a laser which operates in the usual well-known manner.The B particles (chromium) in a ruby crystal have a frequency which istwice the A-particle frequency. The B particle has a forbiddensingle-quantum transition, and decay from the metastable state occurs asa doublequantum or two-photon transition. The presence of the photonsresulting from the two-photon transition causes an avalanche effect anda giant pulse at the priming frequency 11 results. In FIG. 2.. laseractive medium 1 can be doped with B particles which have a transitionfrequency other than twice the priming frequency. The B- particlefrequency may be chosen by careful selection of the host crystal, the Bparticles and reflective layers which present a high-Q to frequencies11,, and 1/ Thus, rather than doping aluminum oxide with chromium toprovide the energizing particle and using a neodymium doped glass toprovide the triggering particles, a system in which the energizingparticle is not at twice the triggering particle frequency may be used.One such system consists of calcium fluoride doped with divalent thuliumas the enerizing B ion and a silicate glass doped with trivalent holmiumas the source of triggering A ions. The difference between the primingfrequency and the B-particle frequency would be the frequency at whichone layer of filter element 3 would be made reflective, the other beingreflective at the priming frequency.

In operation, the laser device of FIG. 2 provides a giant pulse outputat the priming frequency and at a frequency which is different from thepriming frequency. In FIG. 2, excitation lamp 2 which may consist of oneor more flash lamps pumps both the A particles in laser device 7 and theB particles in laser active medium 1 from their normal ground state 12,as shown in FIG. 3, to an excitation level 13 causing a populationinversion of both types of particles. In laser device 7, the triggeringA particles drop from excitation level to a metastable state or level 14as shown in FIG. 3 in a non-radiative transition and from the metastablestate to the terminal state 15 in a radiative transition at frequency 11and, finally from the terminal level 15 to ground state 12 in anonradiative transition. The B particles in laser active medium 1 arealso pumped to some excitation level, but because a single quantumtransition is forbidden, the particles cannot decay to their naturalground state in the same manner as the A particles. In the meantime,photons at frequency 11 are being supplied to laser active medium 1 bylaser device 7 increasing the probability of occurrence of a doublequantum transition. It should be recalled that the nonreflecting orabsorptive property of filter 3 prevents any build up of oscillations atthe B particle frequency. Finally, enough particles are being suppliedto the laser active medium 1 such that an avalanche effect occurs due tothe double-quantum emission phonomenon. Energy stored in laser activemedium 1 is then rapidly converted in radiative transitions intocoherent light outputs at frequencies 11 and 11 in the form ofsimultaneous giant pulses. The energy diagram of FIG. 4 shows therelationship between 1111 hu and hu The fact that hv and hu are shownhaving different energy levels is indicative of the fact that for anyenergy level hv there are a number of possible transitions depending onthe priming frequency which can produce giant pulses at 11A and 11C.

The virtual levels shown in FIG. 4 exist only in the presence of primingphotons at frequency VA. The system behaves as if there were apopulation of real particles with energy levels differing by hv from theoriginal levels but having a concentration proportional to KN S where N=the number of ion/cc. of type B, S,,=the cavity populations of photonsat frequency 11 The analysis which follows makes no specific use ofthese virtual levels, but their apparent presence in the system may behelpful in understanding the physics of the system.

While it is theoretically possible to define the relationship as apractical matter, it should be appreciated that the probability for atwo photon process is broadly resonant at v =v and falls off to zerowhere either 11 or v is greater than 11 FIG. 5 which is a plot ofrelative transi tion probability vs. w /w shows the probability for theoccurrence of a two photon process. Note that where wA/wB is equal to0.5 that the relative transition probability A is a maximum. This is thecase suggested by the article referred to hereinabove where 11 is equalto 211A.

A Q-spoiling technique utilizing 9. Kerr cell is shown in FIG. 6 alongwith the arrangement of FIG. 2. The arrangement of FIG. 6 operates inthe same manner as the device described in connection with FIG. 2 withthe exception that a Kerr cell 16 is interposed between laser 7 andlaser active medium 1. Kerr cell 16 is adapted to cut off or transmitpolarized light in response to the switching of an electrical potentialacross the electrodes 17, 18 of such a cell. The use of a Kerr cellproduces a giant pulse output from laser device 7 thereby enhancing thetriggering conditions which produce the giant pulses at two differentfrequencies.

In FIG. 7, a plot of Power Output vs. Time is shown to indicate therelative timing of the production of giant pulses. Assume that timebegins with the triggering of the excitation of the flash lampsurrounding the laser, and that laser action takes place almostimmediately. This lasing action is due to the presence of the Aparticles and continues during the whole of the given cycle either as anormal laser output or in the form of a giant pulse. The given cycleterminates when the population excess of B particles is largely reduced.Thus, the normal laser spikes at a relatively low power level areproduced during the major portion of the given cycle T. At some time T ahigh energy pulse of light having a very fast rise time occurssimultaneously at frequencies VA and 11 and remains for a minor portionof the given cycle T The high energy pulse at w may have a higher orlower amplitude than that at 11 depending on its frequency which in turndetermines its energy level. Recalling that the summation of thefrequencies M and 11 equal the frequency 11 it is interesting to notethat the summation of the energies in the two simultaneous pulses at 11and w is equal to the power output at a single frequency 11 in the abovementioned article.

It is possible, therefore, to change the power output of a laser activemedium by changing its frequency which is in turn dependent on the B-iondoping, the priming frequency and the Q of the laser cavity.

The following discussion will, it is believed, provide a validtheoretical basis for the teaching that under the proper conditions,giant pulses can be obtained from a laser device at two frequencies suchthat Referring again to the above mentioned article, Some TheoreticalAspects of a Proposed Double Quantum Stimulated Emission Device, by P.P. Sorokin and N. Braslau, calculations were outlined utilizingEquations 1 through 19 which provided the mathematical basis for deviceswhich utilize the two photon transition phenomenon in the case where 11:2

In the present application, Equations 1 through 13 of the Sorokin,Braslau article establish the following rate equations:

of frequency v and v respectively. The cavity decaytime '1' is assumedcommon for the two sets of photons, and the two-photon coupling constantB is C=speed of light in cm. second,

a is a coefficient expressing the strentgth of two-photon absorptiveprocesses,

V=volnme in cubic centimeters,

Mr is the refractive index of the medium.

Regime II.-Devel0pment of the giant pulse In this regime S zS z-S, andthe terms in 1/1- are negligible. Thus Eqs. 1 and 2 above become whichindicates a maximum logarithmic growth rate 1 dS 3 =B1 B( 1 B( which mayfar exceed the logarithmic growth rate of a conventional giant pulselaser, -Eq. 4 would yield the solution showing that the giant pulsetotal growth time is on the order of I/S B N seconds.

O'ES/NB'(O), TEB1N )t, SA=SCES, and remembering that N (t) =N '(0)S,Equation 3 becomes W= BINBZ(O)GZINB(O) with the indefinite integralgiving rise to the plot of FIG. 8.

From the parameters given in the Sorokin-Braslau article N =2 10 and B=3.6 '10- sectthe time unit (B N is found to be 0.7X10 sec., and fromFIG. 8 it is seen that (1) d8 B NB (0) max dt 3X10 sec Such enormousrates of change of population justify the neglect of the 1/7 terms inthis growth regime.

Regime lII.-Decay 1218mm; (7) or which is identical with Eq. 17 shown inthe Sorokin-Br'aslau article except for a trivial factor of 2. Thus if(7) is well satisfied, S will grow exponentially with a time constant1/B S N -(0) until S is no longer small compared to S More precisely,one scheme for priming is to fill the cavity to a level 8 (0) satisfying'(8), and to allow the 11 population to decay freely while the 11population grows. The condition that S S before S decays below thecritical level (8) is thus readily seen to be (considering the initialspontaneous emission from the [N +S *(0)] system into the 11 mode asbeing induced by the zero-point energy of the vacuum).

Strictly speaking, unless S l, Eq. 1 should be written The last termrepresents spontaneous emission and is included in the above analysis bystarting with S =1.

Eq. 9 is obtained as follows: In the priming phase of Method I we haveA( A( and Lgw 9 *i L /L (it S 7' 'r 1' S (9 which integrates directly towhich for S (0)=1, and S (0) /S t/-r 1, gives Eq. 9.

Thus, Method I requires an initial priming photon density about 30 timesas great as is necessary for the degenerate two-photon laser of theSorokin-Braslau article.

Method II.-An alternative to Method I is to supply priming photons 11;;over a period of several cavity decay times. The total number of photons11A and the corresponding supply time required for reaching Regime IIcan thus s -s ln s 10) but that the total number of priming photonsrequired does not increase by much so long as one pumps well over thethreshold (8). Thus, 28 In 5 expresses the number of 1 photons requiredif one maintains a photon level 2S in the cavity for a time 1- ln S -10sec., using the parameters set down in the Sorokin-Braslau article.Normal laser spikes exceed 10- seconds in duration. so that thenondegenerate two-photon laser can be primed by the same photon sourcewhich would be adequate for the degenerate case.

FIGURE 9 shows the course of the various populations as a function oftime, using Method I for priming whereas FIGURE 8 shows the steep regionof the pulse on a time scale expanded -l0 times at the time in FIGURE 9where the populations S and S are equal.

In FIG. 9, the change of population of the photon populations S and Swith time is shown. Note that the population of B ions N remainsconstant until the populations of photons S and S both exceed thethreshold value S At that point, the B ion population N decayssubstantially instantaneously and the photon populations S and S risesubstantially instantaneously to the former value of the B ionpopulation. It should also be noted that the time for the population Sto approach the same population as S is relatively long compared to thetime required for the onset of the giant pulse once S and S have reachedthe same value. This indicates that relative to the devices shown in theSorokin-Braslau article somewhat more power is required by the system ofMethod I to obtain giant pulses.

The predicted performance of a non-degenerate twophoton laser has beenbriefly analyzed indicating the very high logarithmic-growth-rate of thetwo-quantum laser. Normal Q-switched lasers are limited in growth rateby the condition that the cavity in the low-Q status be stable againstthe exponential growth of population in the resonant modes. Thus, if thecavity time constant is switchable between T/ q and 'r, the abovecondition requires the buildup time in the absence of loss to be longerthan 1/ q. For pink ruby, the Q-switched rise time has been shown to beabout 2X10 sec., about three orders of magnitude larger than the risetime calculated above for the two-photon laser.

As has been mentioned herein above, the teaching of this inventioin canbe utilized in connection with solid, liquid or gaseous lasers. The typeof laser is not signifi cant so long as the probability for theoccurrence of multiple photon transitions is large. In this connection,it should be appreciated that multiple photon transitions (i.e., morethan two) can occur, but that the cross-secion or the probability forthe occurrence of a multiple transition is small. One way of increasingthe cross-section in a multiple photon transition system is to providephotons in the form of giant pulses from the devices of FIGS. 1, 2, and6 as a priming frequency. In such an arrangement, only one of the giantpulses can be utilized to trigger a laser device containing particlescapable of a multiple photon transition. The output would be the primingfrequency plus one other frequency, the sum of the output frequenciesbeing equal to the frequency of the particle having the multiple quantumtransition capability. Where the laser device containing particlescapable of multiple photon transitions is triggered by both giant pulseoutputs from the devices of FIGS. 1, 2, and 6, it is possible to obtainoutputs from the laser device at the two priming frequencies and atanother frequency; the summation of these frequencies being equal to thefrequency of the particle having a multiple photon transition.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A laser device comprising a laser active medium containing at least asingle group of particles capable of entering an excited state fromwhich de-excitation occurs as a multiple quantum transition,

means for exciting said at least said single group of particles during agiven cycle,

means optically coupled to said laser active medium reflective at atleast two different frequencies and reflection-free at a frequencycharacteristic of said at least said single group of particles toproduce stimulated emission at one frequency during said given cycle andstimulated emission at at least another frequency and at least a twophoton transition during a portion of said given cycle.

2. A laser device comprising a laser active medium containing at leastone group of particles capable of entering an excited state from whichde-excitation occurs as a multiple quantum transition,

means for exciting said at ticles, and

means optically coupled to said laser active medium reflective at atleast two different frequencies and reflection-free at a sum frequencycharacteristic of said at least said one group of particles.

3. A laser device according to claim 2 wherein said means for excitingsaid at least one group of particles includes a flash lamp opticallycoupled to said laser active medium.

4. A laser device according to claim 2 wherein said means for excitingsaid at least one group of particles includes at least another laseroptically coupled to said laser active medium.

5. A laser device according to claim 2 wherein said means reflective atat least two different frequencies and reflection-free at a sumfrequency characteristic of said at least one group of particlesincludes a filter reflective at said at least two different frequenciesand highly transmissive at said sum frequency.

6. A laser device according to claim 2 wherein said means reflective atat least two different frequencies and reflection-free at a sumfrequency characteristic of said at least one group of particlesincludes a filter reflective at said at least two different frequenciesand highly absorptive at said sum frequency.

7. A laser device comprising a laser active medium containing at leasttwo groups of particles at least one of which is capable of entering anexcited state from which de-excitation occurs as a multiple quantumtransition,

means for exciting said at least said two groups of particles during agiven cycle, and

means optically coupled to said laser active medium reflective at atleast two different frequencies and reflection-free at a frequencycharacteristics of said at least one of said two groups of particles toproduce stimulated emission at one frequency during said given cycle andstimulated emission at at least another frequency and at least a twophoton transition during a portion of said given cycle.

8. A laser device comprising a laser active medium aining at least twogroups of particles at least one of least said one group of par.-

which is capable of entering an excited state from which deexcitationoccurs as a multiple photon transition,

means for exciting said at least two groups of particles,

and means optically coupled to said laser active medium reflective at atleast two different frequencies and reflection-free at a sum frequencycharacteristics of said at least one of said at least two groups ofparticles.

9. A laser device according to claim 8 wherein said means for excitingsaid at least two groups of particles includes a flash lamp opticallycoupled to said laser active medium.

10. A laser device according to claim 8 wherein said means reflective atat least two different frequencies and reflection-free at a sumfrequency characteristic of said at least one of said at least twogroups of particles includes a filter reflective at said at least twodifferent frequencies and highly transmissive at said sum frequency.

11. A laser device according to claim 8 wherein said means reflective atat least two different frequencies and reflection-free at a sumfrequency characteristic of said at least one of said at least twogroups of particles includes a filter reflective at said at least twodifferent fre- 25 quencies and highly absorptive at said sum frequency.

12. A laser device including an exciting source and a laser activemedium optically coupled to said source capable of producing at least atwo photon transition and at least first and second different stimulatedfrequency components characterized by a surface optically coupled tosaid laser active medium which is reflective of a band of frequenciesincluding both said first and second stimulated emission frequencycomponents.

13. A laser device including an exciting source and a laser activemedium optically coupled to said source capable of producing at least atwo photon transition and at least first and second different stimulatedfrequency components characterized by a surface optically coupled tosaid laser active medium which is reflective of a band of frequenciesincluding both said first and second stimulated frequency components andwhich is reflection-free at the sum frequency of said first and secondstimulated frequency components.

14. A laser device comprising a laser active medium containing at leasta single group of particles capable of entering an excited state fromwhich de-excitation occurs as a multiple quantum transition,

means for energizing said laser active medium, and

means optically coupled to said laser active medium for forming anoptical cavity resonant at at least two different frequencies, the sumof which is equal to the characteristic frequency of said at least saidsingle group of particles.

References Cited JEWELL H. PEDERSEN, Primary Examiner.

RONALD L. WIBERT, Assistant Examiner.

